The combination of high power lasers and the efficient confinement of radiation over long lengths available in optical fibers has led to the observation of a variety of nonlinear optical effects. These effects include four-photon processes, stimulated Raman scattering, self-phase modulation and third harmonic generation. These effects are all mediated via the third order susceptibility and are to be expected for centrosymmetric media such as silica-based glasses.
In 1982, Sasaki and Ohmori observed that the injection of intense pulses from a Q-switched and modelocked Nd:YAG laser into a germanium doped fiber resulted in an unexpectedly large amount of second harmonic generation [S. Sasaki and S. Ohmori, J. Opt. Comm. 43 (1983) 83, and S. Sasaki and S. Ohmori, IEEE QU-18 (1982) 758]. Measurements indicated a conversion efficiency of 10.sup.-3, a value that is approximately seven orders of magnitude larger than what is expected from interface and bulk contributions [R. W. Terhune and D. A. Weinberger, J. Opt. Soc. Am B4 (1987) 661]. More recently, Osterberg and Margulis reported that the injection of 1.06 .mu.m pulses into a germanosilicate fiber, co-doped with phosphorus, developed a 5% conversion efficiency over a period of twelve hours [U. Osterberg and W. Margulis, Optics Lett. 11 (1986) 516]. Subsequently, a polarization preserving Ge and P doped fiber has been reported to exhibit a 13% second harmonic conversion efficiency [M. C. Farries, Nonlinear Guided Wave Phenomena: Physics and Applications, 1989 Houston Technical Proceedings, p. 246].
Stolen and Tom reported that the process of fiber preparation could be greatly accelerated if the fibers were simultaneously exposed to both the fundamental and the second harmonic [R. A. Stolen and H. W. K. Tom, Optics Lett. 12 (1988) 584]. These experiments demonstrated that fibers with only Ge as a dopant could be prepared in minutes using this technique, now referred to as seeded preparation. Recently, it has been shown that seeded preparation requires that the fundamental and second harmonic pulses be temporarily coincident, ruling out simple two-photon excitation schemes [M.D. Selker and N. M. Lawandy, OSA Annual Meeting, Orlando Fla. 1989, paper Pd-21]. In addition, experiments on seeded preparation at 77K and 300K revealed no measurable changes in the fiber preparation, indicating that electronic hopping transport does not play a significant role in the process [M. D. Selker and N. M. Lawandy, Electron Lett. 25 (1989) 1440].
However, an efficient second harmonic generation effect has been reported to occur only in drawn fibers. Recent reports correlating self-preparation results with fiber drawing conditions have shown that high drawing tensions result in essentially no second harmonic generation [T. F. Carruthers, C. G. Askins and E. J. Friebel, OSA Annual Meeting, Orlando, Fla., 1989, paper FX4]. This behavior has been attributed to the lack of drawing induced defects in the low tension cases. A model developed by Chen attributes the threshold intensity required at the fundamental to the competition of bulk and interface effects [Y. Chen, App. Phys. Lett 54 (1989) 1195]. However, despite these advances in preparing optical fibers to exhibit second harmonic generation (SHG), heretofore there has been no reported SHG in a bulk silica-based glass, such as a bulk germanosilicate preform of the type from which optical fibers are drawn.